No one is indispensable. However one who is difficult to replace is worth much more than other folks.
Indispensable people go out of the way to get things done. They don't care whether its their duty or whether they have the mandate. They don't wait for the instructions. Going beyond and above their role proactively.They take charge.
These are the qualifiers. You may see many such people around u. Entrepreneurs are the ones but there are many in employed category and they are Worth praising, noticing and following.